Methods for preparing thermoplastic granules by using starches have been disclosed previously (U.S. Pat. Nos. 4,133,784, 4,021,388, DE-A-23 22 440). Such granules are processed to films in a second procedural step. In so doing, the starch is used as filler material in synthetic polymers. Furthermore, methods for preparing and modifying destructured thermoplastic starch have been disclosed (EP-A-378 646, EP-A-397 819).
WO 90/05161 A1 shows thermoplastically processable starch as well as a method for preparing it. In the process aggregates and, if desired, various further additives are added to raw starch material to reduce the melting temperature of the starch to a level below its decomposition temperature. This is to improve the structure of the starch melt so as to facilitate handling in preparing and processing thermoplastic starch.
EP-A1-0 474 095 discloses a process of preparing biodegradable articles, such as dishes, cups, cardboard boxes, etc. using plant-based raw material, such as starch, as starting material. This material is supplemented with additives, plastified and converted in a first extruder to an intermediate to form a billet or granules. Then the intermediate is molded in a second extruder into the desired article.
Similarly, conventional film preparation from plant-based raw materials proceeds in two completely separate steps. The biopolymer melt destructured and plastified in the extrusion process is cooled at the end of the first step, forced through a die under high pressure and granulated. In order to avoid biopolymer foaming, it is required to cool the melt. The resulting high shear stress causes damage to the molecular structure of the starch while various molecule fragments are split off. The polymer chains of the starch are shortened and the molecular weight is reduced. The obtained starch granules are then processed to films in a further extrusion process subject to similar conditions. In the process the biopolymer is again decomposed as the molecular weight is reduced and the chains are shortened. The two-fold damage to the molecular structure, especially the shortening of the amylose chains, causes immediate deterioration of the properties (tensile strength, elasticity, etc.) of the thus obtained films. What is more, the two-fold extrusion is time-, energy- and cost-consuming.
By contrast, the object underlying the present invention is to provide a cost-efficient method for preparing biodegradable films from plant-based raw materials while avoiding the drawbacks of the prior art, improving the quality of the obtained films and achieving, for instance, high tensile strength and elasticity.
This object is achieved by providing the features indicated in the claims.